He will discuss any military action at next week's G8 summit. Whie House officials believe that Congress will vote in favour of strikes against Syria.

Mr Obama said the alleged nerve gas attack which killed more than 1,000 people in rebel-held areas of Damascus earlier this month was "an assault on human dignity" and "a serious danger to our national security".

He went on: "After careful deliberation I have decided that the United States should take action against Syrian regime targets, adding: "Our military has positioned assets in the region."

But he said: "Our capacity to execute this mission is not time-sensitive. It will be effective tomorrow or next week or one month from now.

"I am prepared to give that order.

"I am also mindful that I am the president of the world's oldest constitutional democracy. I will seek approval for the use of force from the American people's representatives in Congress."

Promised to reveal all intelligence

He said there would be a debate and a vote in Congress and promised to give representatives from all parties information about the use of chemical weapons.

The president also called French president Francois Hollande before making Saturday's address.

The US will not wait for UN weapons inspectors to prepare their report and is prepared to act outside the framework of the UN Security Council, the president said.

Mr Obama spoke hours after UN chemical weapons inspectors left Syria for Lebanon, sparking speculation of an imminent US attack.

The United Nations rejected suggestions that it was stepping aside to allow US air strikes and said its humanitarian work in the country would continue.

UN spokesman Martin Nesirky told a press conference: "I have seen all kinds of reporting suggesting that the departure of the chemical weapons team somehow opens a window for military action of some kind.

"Frankly, that's grotesque, and it's also an affront to the more than 1,000 staff, UN staff, who are on the ground in Syria delivering humanitarian aid and who will continue to deliver critical aid."

The Turkish Prime Minister Tayyip Erdogan restated his support for military action outside the UN, saying in a televised address to the nation: "The United Nations Security Council has become a non-functioning body. Therefore, we have decided to act together for Syria, where 100,000 people have been killed."

France is also expected to join the United States in a military coalition, despite an opinion poll which showed that nearly two-thirds of French people oppose military intervention.

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